1. Field of the Invention
The present invention generally relates to an optical information recording and/or reproducing apparatus for recording or reproducing information on or from an optical recording medium and, more particularly, to an optical head assembly designed to obtain various servo signals and reproduced signals by the utilization of rays of light reflected from the optical recording medium.
2. Description of the Prior Art
In order to achieve high-density information recording or reproduction, an optical recording medium, for example, an optical information carrier disc for use with standard optical recording and/or reproducing apparatus, generally contains a spiral or concentric track having a number of track segments of a width not greater than 1 .mu.m and a pitch of 1 to 2 .mu.m. Information recorded on the optical information carrier disc is read therefrom by radiating a beam of light, controlled to a spot diameter of about 1 .mu.m, and then by processing rays of light reflected from the information carrier disc.
The optical information recording and/or reproducing apparatus utilizing the optical information carrier disc of the type referred to above requires at least two servo systems. One servo system is known as a focusing servo system operable to cause the optical head assembly to compensate for a fluttering motion of the information carrier disc in a direction generally perpendicular to the plane of rotation thereof so that the beam spot can be faithfully projected onto the information carrier disc. The other servo system is known as a tracking servo system operable to cause the optical head assembly to compensate for a lateral displacement of the information track in a direction radially thereof such as resulting from an eccentricity of the information carrier disc relative to the axis of rotation thereof so that the beam spot can faithfully pursue the information track.
In general, the servo signal for focusing servo purposes is available by detecting a change in position and/or size of the beam section which would occur when the light beam is defocused. On the other hand, the tracking signal for tracking servo purpose is available by detecting a change in the distribution of the intensity of light reflected from the information carrier disc when the beam spot deviates from the information track on the information carrier disc.
As a means hitherto utilized therefor, use has generally been made of a photodiode having two or more divided sensing areas so that respective amounts of light received by those sensing areas can be compared with each other. In such a case, the position of the photodiode must be properly adjusted so that the center of the beam spot falling on the photodiode when neither focusing error nor tracking error occur may exactly align with the line of division separating the sensing areas from each other. Considering that the photodiode generally employed in the standard optical head assembly has a light receiving area of about 1 mm, with the line of division being about 10 .mu.m, and that the optical head assembly must be adjusted precisely in a range of distance comparable to the thickness of the line of division, the adjustment in position of the optical head assembly is very time-consuming and requires skill.
On the other hand, in the case of a multi-beam head assembly of a type wherein a plurality of laser beams are employed for simultaneous recording and/or reproduction of information on and from plural information tracks, respective points of emission of laser beams from associated multi-beam semiconductor lasers forming a light source are spaced a distance of about several tens of micrometers, with respective axes of the laser beams closely adjoining each other, the use of the conventional method of detecting the amount of all of the light beams by means of the photodiode poses a problem, in that respective spots of the laser beams may overlap with each other on the light receiving area, resulting in a reduction in quality of the signal due to a mix-up of the adjoining laser beams. In order to avoid this, it is necessary to increase the distance of separation between the beams to thereby avoid any possible overlap thereof on the photodiode and, hence, to increase the focal length of a lens used to converge the beams, resulting in an increase in dimension of the head assembly.